Tutorial: Scanner Optical Density Range: DMAX

Optical Density Range

Film has a certain optical density characteristics. These characteristics are used to describe just how "black" dark areas are, how "white" bright areas are, and the range of values between the blackest and whitest parts of the image. There are three labels that describe optical density: D-max (or Dmax), D-min (or Dmin), and D-range (or Drange).

Dmax is the maximum density you can obtain from an image. A film at its very blackest (completely unexposed for slide film, completely overexposed for negative film) has a certain density which is measured as its Dmax. Scanners can only accurately represent a certain range of densities, however. For example, a normal slide film has a Dmax of about 3.5D. However, some slide film can give a Dmax of around 4.0D. If a hypothetical scanner had a Dmax of 3.95D, it would interpret everything denser than 3.95D as completely black. As a general rule, for film slide film usually has a higher Dmax than photo negatives.

Dmin is the density of the whitest parts of the film. A completely translucent material would have a density of 0.0D. However all film has some kind of density, because of the material of the base film. Usually, film has a minimum of about 0.05D. Almost every scanner has a Dmin of 0.0D

Drange measures the difference between the Dmax and Dmin. For example, if a slide film has a Dmin of 0.05D and a Dmax of 3.4D then it has a Drange of 3.35D (3.4D - 0.05D). Drange is an important measure because it indicates a scanner's true capabilities. Dmax can be manipulated by scanner manufacturers to get better specifications for marketing a product. For example, if you set the scanner's light source particularly high, you can get a higher Dmax. However, this causes the translucent (i.e., lower density) areas of film to loose definition (sometimes even turning white and losing all detail). The brighter light source allows the scanner to see detail in dense areas of the film at the expense of detail in light areas. Because of this Drange is often a better indicator of a scanner's quality than Dmax. Scanner manufacturers build advertising around Dmax.

Finally, the scene that you are photographing has inherent optical density characteristics. The Dmax of the scene is the very darkest point in the scene. This is often in shadows. The Dmin of the scene is the very lightest point. If you are taking a picture of a tree, for example, the Dmax might a shadowed point on the trunk or a branch, while the Dmin might be snowy ground, or a wisp of cloud in the sky. The Drange is the difference between those lightest and darkest sources. In the image above, the Dmax is the darkest point on the dial. The Dmin is the lightest point in the background behind the dial.

When editing images, knowing the density range can be helpful because it allows you to match the blacks and whites of your image with the blacks and whites you know are correct. Some photographers, especially when photographing artwork or sculptures, will also take a photo of a color palette or a grayscale palette lit exactly the same way as the artwork, so they know what is white and black in the photograph, and they can match the colors in the photograph with the actual colors.

Why do I care about Dmax?

If you are shopping for a scanner, Dmax is one specification that is often used to claim that one scanner is better (or worse) than another. Don't believe scanner marketing materials! Recognizing that these specifications are subject to manipulation is important. The only way to really evaluate the performance of a particular scanner is to see the results of scanning films which have similar characteristics to film you will scan. Everything else should be taken with a grain of salt!

What Dmax do you really need? If you are scanning negatives, your needs are likely modest. Nearly any film or flatbed scanner has the necessary Dmax to capture all the detail in negative films. In this case, other scanner characteristics are more important (sharpness, speed, cost, etc.). For slide films, Dmax is important. Even typical slide films have densities as high as 3.5D. Generally speaking, slide films tax even the best film scanners. This is one area where drum scanners can excel. (Please read our tutorial on the differences between drum, film and flatbed scanners.)